Optical scanning device

ABSTRACT

An optical scanning device for scanning tracks of an optical record carrier (OD) using a radiation beam, the device including an optical head ( 16 ), defining an optical axis (OA), for converging the radiation beam to a spot when scanning the record carrier; and a rotary arm ( 2 ) for moving the optical head across the record carrier. The device further comprises a rotatably mounted diffraction grating ( 8 ) for generating satellite beams for performing multi-spot tracking. Rotation of the grating is arranged to move the satellite beams, relative to the optical axis of the optical head, in correspondence with rotation of the rotary arm.

This invention relates to an optical scanning device, and in particularto an optical scanning device comprising a rotary arm for scanning anoptical head across an optical record carrier.

The rotary arm system is known for electromechanical adjustment of aread/write head. A rotary arm scanning mechanism is widely used inmagnetic disc recording/reproducing apparatus, commonly known as harddisc drives, for scanning magnetic discs. The use of a rotary arm hasalso been considered for optical disc recording/reproducing apparatus,for scanning optical or magneto-optical discs. A rotary arm provides asimpler mechanism with a reduced number of parts compared to a 2-stagesledge mechanism, which is the most commonly used scanning mechanism inoptical disc systems.

In known optical scanning devices in which a rotary arm scanningmechanism is used, the optical components, including the laser anddetector system, are located on the moving rotary arm. In such a systemall the control and information signals for the laser and the detectorsystem have to be transferred over a connection foil to and from therotary arm system. In the case of a Small Form Factor Optical (SFFO)device, due to the high speeds and the required noise immunity, even theelectronics for driving the laser and processing the detector signalsmay need to be located on the moving arm. This would result in a thermalproblem with heat dissipation of the laser and its associatedelectronics (driver), a dynamical problem due to the relatively heavyweight of the optical and electrical components, and an interconnectionproblem due to the large amount of electrical connections to the lasercircuitry and the detection circuitry.

WO-A-9809285 describes an optical scanning device in which the laserlight is transported to the optical head using optical fibers. A problemwith this approach is the alignment accuracy of the coupling lenses withrespect to the optical fiber. Another problem is the amount of couplinglosses, when the signals are coupled in and out of the fiber.

Another known type of scanning device incorporating a rotary armincludes a periscope. In such a system a set of two mirrors locatedexactly around the rotation axis of the rotary arm will direct the beamto and from the arm. One mirror is fixed to the base plate; the othermirror is fixed to the rotary arm.

U.S. Pat. No. 5,541,908 describes an optical scanning system comprisinga radial arm and a separately mounted optical unit including a radiationsource and a detection system. The radial arm includes a trackingmirror, which is actuated by a one spot push-pull tracking error signal.As is known in the art, a one spot tracking error signal can be of apoor quality in the case of misalignment of the detector with the spot.However, if the preferred three spots tracking were used, rotation ofthe arm would cause movement of the sidebeam spots across the tracks onthe disc, which makes it impossible to generate a tracking error signalcorrectly.

In accordance with the present invention there is provided an opticalscanning device for scanning an optical record carrier using a radiationbeam, the device including:

-   -   an optical head, defining an optical axis, for converging the        radiation beam to a spot when scanning the record carrier; and    -   a rotary arm for moving the optical head across the record        carrier,    -   characterized in that the device further comprises an optical        arrangement for generating satellite beams for performing        multi-spot tracking, and in that the optical arrangement is        arranged to move the satellite beams, relative to said optical        axis of the optical head, in correspondence with rotation of the        rotary arm.

By so moving the satellite beams, the satellite beam spots can bearranged to properly follow tracks on the disc, irrespective of therotary position of the rotary arm.

Further features and advantages of the invention will become apparentfrom the following description of preferred embodiments of theinvention, given by way of example only and with reference to theaccompanying drawings, in which:

FIG. 1 shows a plan view of components of an optical scanning device,arranged in accordance with an embodiment of the invention;

FIG. 2 is a schematic illustration of parameters of the device shown inFIG. 1;

FIG. 3 is a schematic illustration of the positioning of satellite spotsin accordance with an embodiment of the invention when a rotary arm isrotated between different positions;

FIG. 4 is a plan view of a detector array arranged in accordance with anembodiment of the invention; and

FIG. 5 is a plan view of a detector array arranged in accordance with afurther embodiment of the invention.

Referring now to FIG. 1, an optical scanning device in accordance withone embodiment of the invention includes a rotary arm 2 and a separate,fixedly mounted, optical unit 4. The optical unit 4 includes a radiationsource 6, operating at a predetermined frequency, for example afrequency in the region of 400 to 420 nm, projecting a radiation beamalong a first light path LP1 towards a beam splitter 10. Arotatably-mounted three spot diffraction grating 8 is mounted along thefirst light path LP1 between the radiation source 6 and the beamsplitter 10. Note that, hereafter, when reference is made to “theradiation beam”, this generally includes the main radiation beam andeach satellite beam, which are transmitted along similar paths withinthe optical scanning device and through common optical components. Thediffraction grating 8 is a linear diffraction grating, arranged toseparate the light beam into a main, zeroth order beam and first andsecond first order satellite beams to each side thereof. The diffractiongrating 8 is mounted for rotation about a rotation axis coincident withthe center of the first light path LP1. The beam splitter 10 is arrangedto reflect the radiation beam along a second light path LP2, towards acollimator lens 12 which substantially collimates the beam to form abeam consisting of substantially parallel rays of radiation.

A rotary mirror 14 is mounted on the rotary arm 2 adjacent thecollimator lens 12 on the optical unit 4 to receive the radiation beamand direct it along a third light path LP3 towards an optical head 16 atthe far end of the rotary arm 2. In this embodiment, the rotary mirror14 shares an axis of rotation CR with the rotary arm, however in otherembodiments the axes of rotation may be displaced with respect to eachother. The angle of rotation of the rotary mirror 14 is controlled incorrespondence with rotation of the rotary arm 2; generally, the amountof rotation of the rotary mirror 14 will be half of the amount ofrotation of the rotary arm 2. The optical head 16 includes a foldingmirror, arranged at 45° to the third light path LP3, to direct theradiation beam onto an objective lens. The objective lens and thefolding mirror are mounted in a lens holder. This lens holder issuspended on two parallel flexures, which allow the lens holder to movein an axial (focus) direction. A drive coil is located at the front ofthe lens holder. This coil generates axial forces in order to bring thelens to the proper focus. The objective lens has an optical axis OA,along which the main radiation beam is transmitted by the foldingmirror, to form a spot focused, on an information layer of an opticaldisc OD mounted in the optical scanning device, at a location along theoptical axis OA. The satellite beams, meanwhile, pass through theobjective lens to form focused spots on the information layer to eachside of the optical axis OA. The three focused spots are arranged alonga line of alignment having a rotational position about the optical axisOA which is determined by the rotational state of the diffractiongrating 8. The objective lens in the optical head 16 is mounted withinan axial actuator, which is driven by a focus control signal to maintaineach of the main and satellite beam spots in correct focus on theoptical disc OD.

The optical disc OD may be one of a number of different types, includinga read-only type, a recordable (write-once) type and a re-writable type.The optical disc OD is mounted on a spindle in the optical scanningdevice to rotate about a spinning axis SA during scanning of the opticaldisc OD. The optical disc includes data tracks in the form of a pittrain and/or at least one of the lands and grooves of a land/groovestructure, which are arranged on the disc in a spiral or circulararrangement concentric with the spinning axis SA. The optical disc ODmay include one or more information layers; in the case of a multi-layerdisc, the information layers are located at different depths within theoptical disc OD, and the scanning device includes a spherical aberrationcompensation system for compensating the different amounts of sphericalaberration generated when scanning the different information layers.

After scanning of an information layer of the optical disc OD, theradiation beam is reflected back through the optical head 16, along thethird and second light paths LP3, LP2, and in the optical unit 4 istransmitted through the beam splitter 10 to pass along a fourth lightpath LP4 towards a detector array 22. The detector array 22 includesdetector elements, which will be described in greater detail below,which produce a main information signal, a focus error signal and atracking error signal. The focus error signal is used as the focuscontrol signal to drive the focus actuator in the optical head 16,whilst the tracking error signal is arranged to control movement of therotary mirror 14, or the rotary arm 2, as will be described in greaterdetail below. A spot-size focusing type optical element 20, associatedwith a novel detector array 22, to be described in further detail below,is arranged along the fourth light path LP4, to provide a main radiationbeam to the detector array, whereby to conduct spot-size focus errordetection.

The rotary mirror 14 and the rotatable grating 8 may be mounted on arotational bearing consisting of two ball elements on one part, whichmove in a two bearing shale mounted on the other part. The actuation ofthe rotary mirror 14 and the rotatable grating 8 can be performed usinga Lorenz type actuator with a magnet on one part and a coil fixed to anopposing part.

The rotary arm 2 has a bearing system, which consists of two ballelements. One ball element is located on one part; the other ball islocated on the other part. The ball elements are allowed to rotate in aslightly oversized bearing shale. The actuation system for the rotaryarm 2 includes a coil at one end of the arm located in a magnetic field,such that one side of the coil is in a vertical upwards field and theother side of the coil is in a vertical downwards field. A currentthrough the coil will generate Lorenz forces, which rotate the arm inthe desired direction.

A two-stage tracking control method is applied where one of the rotaryarm 2 or the rotary mirror 14 is controlled via a separate sensor signalfrom a detector on the rotary arm or from a signal reflected from theoptical media, and the other of the rotary arm 2 or the rotary mirror 14is controlled via the detected tracking error signal generated in thedetector array 22.

The separate sensor signal can be provided using two forward sensephotodiodes positioned on the rotary arm at both sides of the radiationbeam. Whenever the beam is not positioned correctly, one of the twophotodiodes will collect more light than the other, providing a welldefined control signal. Alternatively, the separate sensor signal can begenerated by sampling a blank part on the media. A shift of the beam inthe pupil of the objective lens will generate a shift in the center ofthe reflected beam. This shift can be detected via a differentialdetection on the detector array 22.

There are two possible actuator control arrangements possible for aproper cooperation of the rotary mirror 14 with the rotating arm. In oneembodiment the mirror is actuated as a slave, the rotary arm is actuatedas a master. The rotary mirror 14 is driven by the separate sensorsignal as generated by one of the above mentioned methods. The rotaryarm 2 is driven by the tracking error signal generated in the detectorarray 22. The bandwidth of the arm actuator could be up to 2-3 kHz. Thebandwidth of the mirror actuator is preferable approximately a factor 10lower, say 200-300 Hz.

In a second actuator control arrangement the rotary arm is actuated as aslave, the mirror is actuated as a master. The rotary mirror 14 isdriven by the tracking error signal generated in the detector array 22.The rotary arm 2 is driven by the separate sensor signal as generated byone of the above mentioned methods. An advantage of this type of loop isthat it can utilize the high bandwidth, which is possible with a smalltracking mirror. The bandwidth of total system is not limited by thelarger arm construction. The bandwidth of the mirror actuator can besome 5-6 kHz, the arm actuator bandwidth some 300-400 Hz.

FIG. 2 is a schematic illustration of parameters of the arrangement ofthe optical head 16 on the rotary arm 2 with respect to the disc OD, andthe direction of tracks on the disc OD. An arbitrary x-y coordinatesystem is chosen with the disc positioned with the center of the tracks,SA, at coordinates (0,0). The rotary arm 2 has a length L between therotation axis CR and the optical axis OA of the optical head 16. Therotation axis CR is located at coordinates (x₀,y₀). The angle of therotary arm with respect to the x-axis is denoted α. At a certain angle αthe laser is focussed at point OA on the disc. OA is described by aradius r and an azimuthal angle φ (with respect to the positive x-axis).The angle of the tracks at point OA with respect to the positive x-axisis therefore given by φ+π/2. For a given arm length L, φ and r can beexpressed as function of the rotary arm (angular) position a. Namely,φ(α) and r(α) can be written as: $\begin{matrix}{{r(\alpha)} = \sqrt{\left( {x_{0} + {L \cdot {\cos(\alpha)}}} \right)^{2} + \left( {y_{0} + {L \cdot {\sin(\alpha)}}} \right)^{2}}} & (1) \\{{\phi(\alpha)} = {\arctan\left( \frac{y_{0} + {L \cdot {\sin(\alpha)}}}{x_{0} + {L \cdot {\cos(\alpha)}}} \right)}} & (2) \\{{\delta(\alpha)} = {{\phi(\alpha)} - \alpha + {\pi/2}}} & (3)\end{matrix}$

As the arm rotates to move the optical head 16 across the disc theangles φ and α change, and consequently the angle δ, the orientation ofthe rotary arm with respect to the track direction, changes. Theorientation of the rotary arm is equal to the orientation of anarbitrary line, passing through the optical axis OA and aligned parallelwith the rotary arm, on the optical head 16. By rotation of the grating8 by an amount equal to that angle (δ) the position of the spots on thedisc with respect to the arbitrary line on the optical head 16 changescorrespondingly and the alignment of the spots on the disc with respectto the track direction is maintained.

FIG. 3 illustrates the way in which the alignment of the beam spots onthe optical disc OD is controlled during rotation of the rotary arm 2 bycorresponding rotation of the diffraction grating 8. FIG. 3 shows thepositioning of the spots in first and second scanning areas A, B, eachshown in magnified plan view in FIG. 3, of the optical disc OD, when therotary arm 2 is rotated between a first position LP3 and a secondposition LP3′, which are separated by an angle of rotation a about thecenter of rotation CR. In the first position, for scanning in the firstscanning area A, the main spot 30 follows a path centered on one of thedata track sections 40 of the optical disc OD. Meanwhile, the satellitebeams 32, 34 each follow a path halfway between two respective adjacenttrack sections 40. The path followed by the satellite beam spots 32, 34is thus laterally separated from the path followed by the main beam spot30 by ±(N+½)tp, where N is an integer (0, 1, 2 . . . ) and where tp isthe track pitch. The rotational position of the grating 8 is controlledsuch that the three spots 30, 32, 34 are arranged along a line ofalignment 36, which is perpendicular to the direction of the tracksections 40 in the scanning area A.

In the second rotary position of the rotary arm 2, for scanning in thesecond scanning area B, the track sections 40′ are arranged at an anglewith respect to the track sections 40 in the first scanning area A. Therelative angular separation between the first data track sections 40 andthe second data track sections 40′ is a function of the relativepositioning of the spinning axis SA of the disc OD and the center ofrotation CR of the rotary arm 2, along with the length of the rotary arm2 between the center of rotation CR and the optical axis OA of theoptical head 16.

In the second rotary position, the diffraction grating 8 is rotated,with respect to its position in the first position of the rotary arm, bythe angle δ(α), which produces a corresponding and equal rotation of thesatellite spots 32′, 34′ with respect to the optical axis OA of theoptical head 16 of δ(α). The amount of rotation of the satellite spots32′, 34′ is controlled in correspondence with rotation of the rotary armto ensure that the line 36′ along which all three spots 30′, 32′, 34′are aligned remains perpendicular to the data track sections 40′. Theamount of rotation takes into account firstly the angle of rotation α ofthe rotary arm 2 and secondly the angular separation of the data tracksections between the first scanning position and the second scanningposition, according to the relationships given in equations (2) and (3)above. Thus, δ(α) varies in relation to a by an amount which takes intoaccount the amount of angular separation between the track sections 40in the first scanning position and the track sections 40′ in the secondscanning position on the optical disc.

FIG. 4 illustrates a detector array arrangement suitable for detecting amain information signal, a spot-size focus error signal and a threespots push pull tracking error signal from two main beam spots 50, 51and a set of satellite beam spots 52, 54 at the detector. One set ofthree beam spots 50, 52, 54 are aligned along a line 56 which rotatesabout the main beam spot 50 in correspondence with the rotation of thelines of alignment 36, 36′, along which the scanning beam spots arearranged, with respect to the optical head 16. A further set of threebeam spots 51, 53, 55 are aligned along a similar line 57.

Conventional main spot detectors 60, 61, each including for example athree segment detector arrangement for e.g. spot-size detection, asknown in the art, is used to detect information and control signals fromthe main beam spots 50, 51, including a main data signal, a spot-sizefocus error signal and a component of the three spot push pull trackingerror signal, as is known in the art. Two novel satellite beam spotdetectors 62, 64 are arranged to each side of one of the main beam spotdetectors 60. The satellite beam spot detectors each include twodetector segments 66, 68; 72, 74 separated by a separating line 70; 76which is arranged to bisect each satellite beam spot 52; 54 irrespectiveof the angular position of the line of alignment 56. Thus, eachseparating line 70; 76 is curved in a manner such that the center of thesatellite beam spot coincides with a point along the separating line 70,76 as the satellite beam spots 52, 54 are rotated at the main beam spot50 due to rotation of the diffraction grating 8 during rotation of therotary arm 2. The push pull tracking error signals from each of thethree detectors 60, 62, 64 are combined in a known manner, to form acombined three spot push pull tracking error signal, which is wellcompensated for misalignment between the detector array and the beamspots.

FIG. 5 illustrates an alternative detector array arrangement suitablefor detecting a main information signal, a spot-size focus error signaland a three spots push pull tracking error signal. The arrangement issimilar to that shown in FIG. 4, and similar features are referencedwith the same reference numerals, except incremented by 100; thedescription thereof above should be taken to apply here also. In thisembodiment, the separating lines 170, 176 are straight rather thancurved, which increases ease of manufacture. Even in this embodiment,the separating lines 170, 176 are generally aligned with a direction ofmovement of the satellite detector spots when the rotary arm is rotatedProviding the angle of rotation of the rotary arm is not too large, thisarrangement of detector elements can provide a reliable tracking errorsignal, as the movement of the spots to each side of the separatinglines as the satellite spots rotate is symmetrical and is thereforecompensated in the push-pull signal processing method.

The above embodiments are to be understood as illustrative examples ofthe invention. Further embodiments of the invention are envisaged. Forexample, whilst the above-described embodiment uses a spot-size focuserror detection method, alternatively a Foucault type focus errordetection method may alternatively be used in combination with a threespot tracking error detection method.

It is to be understood that any feature described in relation to any oneembodiment may be used alone, or in combination with other featuresdescribed, and may also be used in combination with one or more featuresof any other of the embodiments, or any combination of any other of theembodiments. Furthermore, equivalents and modifications not describedabove may also be employed without departing from the scope of theinvention, which is defined in the accompanying claims.

1. An optical scanning device for scanning tracks of an optical recordcarrier using a radiation beam, the device including: an optical head,defining an optical axis, for converging the radiation beam to a spotwhen scanning the record carrier; and a rotary arm for moving theoptical head across the record carrier, characterized in that the devicefurther comprises an optical arrangement for generating satellite beamsfor performing multi-spot tracking, and in that the optical arrangementis arranged to move the satellite beams, relative to said optical axisof the optical head, in correspondence with rotation of the rotary arm.2. An optical scanning device according to claim 1, wherein the opticalarrangement is arranged to rotate the satellite beams, relative to saidoptical axis of the optical head, in correspondence with rotation of therotary arm.
 3. An optical scanning device according to claim 1, whereinthe optical arrangement comprises means for generating the satellitebeams, and said means are rotatable to provide said movement of thesatellite beams.
 4. An optical scanning device according to claim 3,wherein said means comprise a diffraction grating.
 5. An opticalscanning device according to claim 1, wherein said device comprises aradiation source mounted separately from said rotary arm.
 6. An opticalscanning device according to claim 1, wherein said device comprises adetector array comprising satellite detector elements arranged to detectsatellite detector spots in a range of rotational positions about a maindetector spot.
 7. An optical scanning device according to claim 6,wherein said satellite detector elements comprise two elements separatedby a separation line which is generally aligned with a direction ofmovement of the satellite detector spots when the rotary arm is rotated.8. An optical scanning device according to claim 6, wherein said devicecomprises spot-size type or Foucault type optical element associatedwith said detector array.
 9. An optical scanning device according toclaim 1, wherein the optical arrangement is arranged to move thesatellite beams between a first positioning, in a first rotary positionof the arm and a second positioning, in a second rotary position of thearm, so as to take into account rotary movement of the arm relative tothe record carrier.
 10. An optical scanning device according to claim 1,wherein the record carrier is in the form of an optical disc and theoptical arrangement is arranged to move the satellite beams between afirst positioning, in a first rotary position of the arm and a secondpositioning, in a second rotary position of the arm, so as to take intoaccount angular separation of the tracks, relative to each other, at theoptical head in the first and second positions of the rotary arm.
 11. Anoptical scanning device according to claim 1, wherein the opticalarrangement is arranged to position satellite beam spots on the recordcarrier along a line which is substantially perpendicular to thedirection of the tracks.